The invention relates to digitally programmed frequency synthesizers and, more particularly, those which include at least one first phase control loop generating small frequency steps, a second loop generating greater steps, and a third loop arranged in such a way as to give the algebraic sum of the frequencies generated by the two first loops.
Such synthesizers allow discrete frequency scanning, step by step, over a range very much greater than that which is practicable with a single loop and that with good resolution and with a satisfactorily pure waveform. They can thus, in particular, be used in the construction of programmable wobbulators or generators in which the frequency is set manually, for example, by means of a knob.
Known circuits of this type, however, have considerable drawbacks.
The first results from the fact that the oscillator of the first loop returns abruptly to its frequency of origin every time the frequency of the oscillator of the second loop progresses one step. As it is impossible in practice to achieve strict synchronism between the return of the first oscillator to its frequency of origin and the step of the second oscillator, and as the times required for the two oscillators to come together are not identical, there is a discontinuity during the transition between the large steps which can be considerable in the increase in the output frequency from the synthesizer.
The second drawback reveals itself when it is desired to reduce the importance of the first, which is done by constructing the control loops in such a way that their transient times are very short. The jump in frequency which is apparent during transition is then much reduced, but more abrupt, and when it is passed through a circuit with limited passband (which is the case when there are filters present), there results a damped oscillation of a frequency equal to the value of the jump. In other words the synthesized signal is amplitude-modulated, which is extremely troublesome.